Although the economical synthesis of pale-colored anionic surfactants is nowadays definite state of the art, during the production and the processing of such surfactants, problems relating to applications arise. For example, the anionic surfactants are produced in the course of the production method in their acid form and have to be converted to their alkaline metal or alkaline earth metal salts using suitable neutralizing agents.
This neutralization step can be carried out with solutions of alkali metal hydroxides or else with solid alkaline substances, in particular sodium carbonate. In the case of neutralization with aqueous alkalis, the surfactant salts are produced in the form of aqueous preparation forms, it being possible to establish water contents in the range from about 10 to 80% by weight and in particular in the range from about 35 to 60% by weight. Products of this type have a paste-like to cutable nature at room temperature, the flowability and pumpability of such pastes being limited or even lost even in the region of about 50% by weight of active substance, giving rise to considerable problems during the processing of such pastes, in particular during their incorporation into solid mixtures, for example into solid detergents and cleaners. Accordingly, it is an old requirement to be able to make available anionic detergent surfactants in dry, in particular pourable, form. In fact, it is also possible to obtain pourable anionic surfactant powders or granulates, in particular those of fatty alcohol sulfates (FAS) by conventional drying technology. However, there are serious limitations here since the resulting preparations are often hygroscopic, absorb water from the air during storage to form clumps and even in the finished detergent product have a tendency toward clumping.
Comparable or other difficulties arise during the conversion of aqueous, in particular paste-like, preparation forms of numerous other washing- and cleaning-active surfactant compounds to give storage-stable solids. Further examples of anion-active fatty chemical surfactant compounds to be mentioned are the known sulfo fatty acid methyl esters (fatty acid methyl ester sulfonates, MESs), which are prepared by α-sulfonation of the methyl esters of fatty acids of vegetable or animal origin with predominantly 10 to 20 carbon atoms in the fatty acid molecule and subsequent neutralization to give water-soluble mono salts, in particular the corresponding alkali metal salts. As a result of ester cleavage, they produce the corresponding sulfo fatty acids or their disalts which, like mixtures of disalts and sulfo fatty acid methyl ester monosalts, are attributed important properties with regard to washing and cleaning. Finally, however, even drying of an aqueous paste of the alkali metal salts of washing-active soaps and/or ABS pastes can also be accompanied by considerable problems.
An alternative to the spray-drying of surfactant pastes is granulation. The patent literature also contains broad prior art relating to the non-tower production of detergent and cleaners. Many of these processes start from the acid form of the anionic surfactants since this class of surfactant represents the largest fraction of washing-active substances in terms of amount, and the anionic surfactants are produced in the course of their preparation in the form of the free acids, which have to be neutralized to the corresponding salts.
For example, European patent application EP-A-0 678 573 (Procter & Gamble) describes a method for producing pourable surfactant granulates with bulk densities above 600 g/l, in which anionic surfactant acids are reacted with an excess of neutralizing agent to give a paste with at least 40% by weight of surfactant, and this paste is mixed with one or more powder(s), of which at least one must be spray-dried and comprises the anionic polymer and cationic surfactant, where the resulting granulate may be optionally dried. Although this specification reduces the fraction of spray-dried granulates in the detergents and cleaners, it does not avoid spray-drying entirely.
European patent application EP-A-0 438 320 (Unilever) discloses a batch process for producing surfactant granulates with bulk densities above 650 g/l. In this process, a solution of an alkaline inorganic substance in water, with the possible addition of other solids, is admixed with the anionic surfactant acid and granulated with a liquid binder in a high-speed mixer/granulator. Although neutralization and granulation take place in the same apparatus, they are in separate process steps, meaning that the process can only be operated batchwise.
European patent application EP-A-0 402 112 (Procter & Gamble) discloses a continuous neutralization/granulation process for producing FAS and/or ABS granulates from the acid in which the ABS acid is neutralized with at least 62% strength NaOH and then granulated, with the addition of auxiliaries, for example ethoxylated alcohols or alkylphenols or a polyethylene glycol with a molar mass between 4000 and 50 000 which melts above 48.9° C.
European patent application EP-A-0 508 543 (Procter & Gamble) gives a process in which a surfactant acid is neutralized with an excess of alkali to give an at least 40% strength by weight surfactant paste, which is then conditioned and granulated, direct cooling taking place with dry ice or liquid nitrogen.
Dry neutralization processes in which sulfonic acids are neutralized and granulated are disclosed in EP 555 622 (Procter & Gamble). According to the teaching of this specification, the neutralization of the anionic surfactant acids takes place in a high-speed mixer by means of an excess of finely divided neutralizing agent with an average particle size below 5 μm.
A similar process which is also carried out in a high-speed mixer and in which sodium carbonate ground to 2 to 20 μm is used as neutralizing agent is described in WO 98/20104 (Procter & Gamble).
Surfactant mixtures which are subsequently sprayed onto solid absorbers and provide detergent compositions or components therefor are also described in EP 265 203 (Unilever). The liquid surfactant mixtures disclosed in this specification comprise sodium or potassium salts of alkylbenzenesulfonic acids or alkylsulfuric acids in amounts up to 80% by weight, ethoxylated nonionic surfactants in amounts up to 80% by weight, and at most 10% by weight of water.
Similar surfactant mixtures are also disclosed in the earlier EP 211 493 (Unilever). According to the teaching of this specification, the surfactant mixtures to be sprayed on comprise between 40 and 92% by weight of a surfactant mixture, and more than 8 to at most 60% by weight of water. The surfactant mixture consists in turn of at least 50% polyalkoxylated nonionic surfactants and ionic surfactants.
A process for producing a liquid surfactant mixture from the three constituents anionic surfactant, nonionic surfactant and water is described in EP 507 402 (Unilever). The surfactant mixtures disclosed here, which reportedly comprise little water, are produced by bringing together equimolar amounts of neutralizing agent and anionic surfactant acid in the presence of nonionic surfactant.
German laid-open specification DE-A-42 32 874 (Henkel KGaA) discloses a process for producing washing- and cleaning-active anionic surfactant granulates by neutralizing anionic surfactants in their acid form. The neutralizing agents disclosed here are solid, pulverulent substances, in particular sodium carbonate which reacts with the anionic surfactant acids to give anionic surfactant, carbon dioxide and water. The resulting granulates have surfactant contents around 30% by weight and bulk densities of less than 550 g/l.
European laid-open specification EP 642 576 (Henkel KGaA) describes a two-stage granulation in two serially connected mixers/granulators, where, in a first, low-speed granulator, 40–100% by weight, based on the total amount of the constituents used, of the solid and liquid constituents are pregranulated and, in a second, high-speed granulator, the pregranulate, optionally with the remaining constituents, is mixed and converted to a granulate.
European patent specification EP 772 674 (Henkel KGaA) describes a process for producing surfactant granulates by spray-drying, in which anionic surfactant acid(s) and high-concentration alkaline solutions are supplied separately with a gaseous medium and mixed in a multicomponent nozzle, neutralized and spray-dried by spraying into a stream of hot gas. The finely divided surfactant particles obtained in this way are then agglomerated in a mixer to give granulates with bulk densities above 400 g/l.
German laid-open specification DE-A-43 14 885 (Süd-Chemie) discloses a process for producing washing- and cleaning-active anionic surfactant granulates by neutralization of the acid form of anionic surfactants with a basic-acting compound, the hydrolysis-sensitive acid form of an hydrolysis-sensitive anionic surfactant being reacted with the neutralizing agent without the liberation of water. Preference is given to using sodium carbonate as neutralizing agent; it reacts in this process to give sodium hydrogencarbonate.